What is a reasonable explanation for the gap between the quarters with weights between 5.5 grams and

Question:

What is a reasonable explanation for the gap between the quarters with weights between 5.5 grams and 5.8 grams and the group of quarters with weights between 6.0 grams and 6.4 grams?
Required
Answer the questions by referring to the following Minitab-generated histogram, which depicts the weights (grams) of all quarters listed in Data Set 29 "Coin Weights" in Appendix B. (Grams are actually units of mass and the values shown on the horizontal scale are rounded.)